An optical method is known for measuring without contact the diameter of an optical fiber running through a laser beam. Measurement accuracy is about .+-.0.2 .mu.m. Such a method could be considered for deposits having a thickness of more than 1 .mu.m, but it is unsuitable for the thicknesses for which the present invention is intended.
U.S. Pat. No. 4,952,226 describes a method of that kind based on detecting laser light diffracted by a fiber, but the result of such measurement is greatly disturbed by lateral displacements of the fiber, which displacements are practically unavoidable in a fiber-drawing machine.
Commercially available measuring apparatuses are also known based on inducing eddy currents at very high frequency. Such apparatuses enable thicknesses greater than 5 .mu.m to be measured on objects having a diameter of not less than 1 mm. Extrapolating the method implemented in such apparatuses to objects of smaller diameter provided with thinner deposits is not technically feasible.
An object of the present invention is to provide industrial apparatus enabling measurement to be performed without contact on a running fiber while preserving the mechanical strength of the fiber.